Suspension systems of the type described are known from, for example, JP-A-2-57413. A suspension system disclosed in JP-A-2-57413 is provided as a double wishbone suspension including upper and lower arms retaining knuckles in such a manner as to allow pivotal movements of the knuckles. The suspension system of this type will be explained with reference to FIG. 16.
FIG. 16 is a view schematically showing a right half of a vehicle body 201 of a vehicle 200 including conventional right and left double wishbone suspensions (only right one shown and designated at 210).
The suspension 210 suspends a right wheel 214 from the vehicle body 201. The suspension 210 includes upper and lower arms 211, 212, connected to a right side of the vehicle body 201 in such a manner as to pivot up and down, and a knuckle 213 connected to respective distal ends of the upper and lower arms 211, 212.
When the vehicle 200 turns leftward, a damper or a spring of the right suspension 210 is compressed while a damper or a spring of the left suspension is stretched because the vehicle 200 is subjected to a centrifugal force. As a result, the vehicle 200 causes a body roll, namely, the vehicle body 201 leans rightward such that the right side of the vehicle body 201 is lowered while a left side of the vehicle body 201 is lifted up. It is to be noted that, when the vehicle 200 turns leftward, the right wheel 214 is an outside wheel while a left wheel (not shown) is an inside wheel.
The body roll of the vehicle 200 is geometrically defined by arrangement of suspension links (suspension arms). The vehicle body 201 rolls on a roll center RC2.
As shown in FIG. 16, the right wheel 214 has a contact point Pg2 contacting a ground surface GL. A roll center height Hr2 is defined as a distance between the ground surface GL and the roll center RC2. A line Li2 passes through the point Pg2 and the roll center RC2. The line Li2 is inclined at an angle θ2 relative to the ground surface GL.
When the right wheel 214 pivots upward relative to the vehicle body 201, the contact point Pg2 of the right wheel 214 is shifted laterally outwardly of the vehicle body 201. The contact point Pg2 then moves about the roll center RC2 in an arcuate line Lg2 extending upwardly outwardly of the vehicle body 201. This results in relatively great roll center height Hr2.
A cornering force (sideways force) CF2 is produced at the contact point Pg2 of the right wheel 214. The cornering force CF2 includes a component (called “horizontal component”) Fs2 directed towards the roll center RC2 and a component (called “vertical component”) Fg2 directed towards the ground surface GL. These two components Fs2, Fg2 each correspond in magnitude to the angle θ2. The component Fs2 is applied to the suspension 210. A reaction force of the component Fg2 acts in such a direction as to lift up the wheel 214. The component Fg2 can be generally expressed by the following equation:Fg2=CF2×sin θ2.
Since the angle θ2 is relatively great, the component Fg2 is great, too. That is, the reaction force becomes large. It is necessary to reduce the reaction force acting in such a direction as to lift up the wheel 214.
The suspension 210 is disposed to provide a wheel alignment designed such that a tire of the right wheel 214 is disposed perpendicular to the ground surface GL. In other words, the wheel 214 is arranged to provide an increased negative camber when the suspension 210 shifts upwardly.
However, since links of the double wishbone suspension 210 are arranged in such a manner as to provide the increased negative camber of the wheel 214, the roll center height Hr2 is rendered large. This results in a large component Fg2. That is, a large reaction force of the large component Fg2 acts on the right wheel 26 in such a direction as to lift up the right wheel 26. Thus, a pressure applied from the ground surface GL to the tire of the right (outside) wheel 214 is abruptly increased while a pressure applied from the ground surface GL to a tire of the left (inside) wheel is abruptly decreased. As a result, a load applied to the right wheel 214 is abruptly increased. A total grip of the tires of the right and left wheels would be abruptly decreased since the tire has a property of providing a very small grip when an excessively large load is applied to the tire. Thus, it is difficult to provide optimized tire grips. Moreover, it is difficult to improve a damping function of the suspension 210 when the vehicle 200 makes a left turn.
The reason why the above problem occurs is that, when the suspension 210 shifts upwardly, the upper arm 211 draws an upper part of the tire of the wheel 214 inwardly of the vehicle body 201 while the lower arm 212 retains or pushes a substantially central portion of the tire of the wheel 214 outwardly of the vehicle body 201 such that the contact point Pg2 of the wheel 214 is displaced laterally outwardly of the vehicle body 201 and then moves in the acuate line Lg2.
To address the problem, there is a need to provide: (1) a suitable camber for efficient contact between a wheel and a ground surface; and (2) an appropriate roll center height determining how much a load applied to each of right and left wheels is varied during turning of a vehicle.